Analysis of Crack Evolution Characteristics and Damage Assessment of Slabs with Openings Based on Fractal Theory

To accurately and rapidly assess the damage level of slabs with openings in subway stations, a graded earth pressure loading device was developed. Model experiments revealed the crack evolution characteristics on the concrete surface of slabs with openings under graded earth pressure. A damage assessment method based on fractal theory was proposed, and empirical equations were established linking the fractal dimension with slab deflection, static stiffness, and the damage index. The results demonstrate that the damage process of opening slab structures can be divided into four stages: initial damage accumulation, damage manifestation, damage intensification, and damage saturation. The fractal dimension effectively characterizes the crack evolution features and damage states of the concrete surface in opening slabs. Under varying earth pressure levels, regions with the same opening area exhibit a linear increase in fractal dimension as earth pressure levels rise. During the graded earth pressure loading process, the fractal dimension of cracks in the slab opening region ranges from 1.45 to 1.88. Exponential relationships were identified between the fractal dimension of opening slabs and their mid-span deflection, static stiffness, and damage indices. This approach provides a novel method for rapidly evaluating the damage level of opening slabs based on fractal dimension analysis.